1. Field of the Invention
The present invention relates to a flat gasket made of a soft material, which gasket is useful as an exhaust flange gasket, a cylinder head gasket or a secondary gasket for internal-combustion engines, and is composed of a porous soft material, optionally containing fillers and binders, which porous soft material is impregnated by a liquid silicone resin and is crosslinked in the gasket when it is ready to be installed.
2. Description of the Related Art
In practice, soft material cylinder head gas kets are usually composed of fiber web materials which are optionally reinforced with metal. While asbestos fibers were used previously as a fiber web material, at present it is preferable to use fiber webs which are more or less free of asbestos and are generally mixtures of fibers of organic or inorganic and synthetic or natural origin and sealing agent components and, optionally, fillers.
Graphite gaskets have been put to use as asbestos-free cylinder head gasket materials. These prior art graphite gaskets are composed of graphite sheet and sheet metal laminates or of metal plates onto which graphite has been pressed and which, optionally, also contain binders, that is graphite binder sheets. However, pure graphite gaskets have the drawback that they cling to the sealing faces of the internal-combustion engine.
Soft material cylinder head gaskets for internal-combustion engines generally are impregnated, primarily to improve their service life, sealing quality and strength. According to U.S. Pat. No. 3,970,322, useful impregnating agents are composed of crosslinkable liquid organic-synthetic substances which are thermally crosslinked in the gasket when it is ready to be installed.
Liquid silicone resin systems are known which preferably crosslink catalytically when metal salts are added. Such silicone resin systems have been used to impregnate soft substance gaskets and, because of their excellent technological characteristics, these gaskets exhibit a particularly good resistance to the media to be sealed, to temperature influences and to aging.
According to German Published Application DE-OS No. 3,245,664, which corresponds to U.S. Pat. No. 4,499,135, a polymethyl vinyl siloxane resin is used as the liquid silicone resin which crosslinks with platinum complex salts during polyaddition. This crosslinking system, however, is sensitive to amine compounds which act to poison the catalyst and to sulfur-discharging compounds so that sulfur-free and amine-free sealing plates must be used. However, due to the nature of the rubber binders generally used in sealing materials, production of sealing plates which are free of amines and sulfur-discharging compounds is possible only at great expense.
According to German Published Application DE-OS No. 3,317,501, which corresponds to U.S. Pat. Nos. 4,483,539 and 4,500,100, liquid polysiloxanes having crosslinkable hydrogen groups, such as methyl hydrogen polysiloxanes, are used to impregnate gasket preforms. Organic heavy metal salts are added as crosslinking catalysts. After crosslinking, however, such impregnated gaskets still contain reactive hydrogen groups from the polysiloxane, particularly in the surface regions thereof, so that, during storage and transport, the gaskets very disadvantageously stick to the packaging material and, after installation, to the sealing faces of the internal-combustion engines. These gaskets are therefore generally additionally coated with a condensation crosslinking polysiloxane resin and, once the coating has crosslinked, such gaskets no longer cling, but the process of manufacturing such gaskets is complicated and expensive and, thus, uneconomical.